Our long-term goal is to understand intestinal epithelial transport and barrier function. Substantial evidence suggests that these critical functions are altered in the setting of inflammatory bowel disease and other intestinal disorders. Moreover, we hypothesize that inflammatory bowel disease results when intestinal barrier function is inadequate to exclude luminal factors, thereby resulting in immune stimulation and a vicious cycle of inflammation and injury that further compromises transport and barrier homeostasis. We will also test the specific hypothesis that probiotic microorganisms exert protective effects on intestinal epithelial cells under both basal conditions and when they are compromised by pathogenic microorganisms or other injurious insults. We will test our hypotheses by addressing two Specific Aims. In the first, we will examine whether barrier and transport functions are altered in a mouse model of inflammatory bowel disease known to depend on an epithelial defect, and determine how such alterations contribute to the pathogenesis of inflammation and/or the generation of symptomatology. The interplay with luminal bacteria will also be sought. In the second Aim, we will define mechanisms whereby probiotic microorganisms improve epithelial barrier and transport function under control conditions, when these functions are compromised in vitro, and in the model of inflammatory bowel disease described above. The approach will be to use the mdr1a -/- mouse which develops spontaneous colitis when conventionally housed, as well as human intestinal epithelial cell lines. We will also employ pathogenic bacteria and two representative probiotic strains. We will study transport and barrier functions in these models using electrophysiological approaches, and will apply molecular techniques to define protein targets of inflammation, or which underlie the beneficial effects of probiotics. The significance of this work lies in its potential to yield novel insights into the pathogenesis of inflammatory bowel disease. It may also provide a scientific rationale for continued exploration of probiotics, promising complementary/alternative therapies, in the treatment of inflammatory bowel disease and other intestinal disorders.